Depiction of how the dual-PSAPD modules are stacked. (a) Depiction of one dual-PSAPD module, which consists of two scintillation crystal arrays and two PSAPDs configured two units deep with respect to incoming 511 keV photons, mounted directly on a flex circuit. (b) A stack of dual-PSAPD modules with 511 keV photons entering edge-on. The design enables 1 mm3 intrinsic resolution, directly measured 1 mm DOI resolution, the capability to position the 3D interaction coordinates of multiple interaction photon events, and >90% scintillation light collection efficiency, independent of interaction location.